MAGNETIC ENCODER WITH IMPROVED RESOLUTION
A low-cost magnetic encoder that facilitates generating sinusoidal magnetic flux is provided. First and second permanent magnet arrays each include a plurality of permanent magnets arranged such that magnetic poles having the same polarity face each other, and magnetic yokes disposed on side surfaces of the plurality of permanent magnets. The permanent magnets and the magnetic yokes are arranged side by side at a predetermined pitch in the moving direction of a magnetic piece array. First and second magnetic detectors corresponding to the first and second permanent magnet arrays are disposed in a positional relationship allowing detection of leakage magnetic flux generated when the permanent magnet arrays and the magnetic piece array are displaced with respect to each other.
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The present invention relates to a magnetic encoder.
BACKGROUND ARTMagnetic encoders including magnets and magnetic detecting elements are used. In the magnetic encoders which use only magnets, however, an error due to the magnets is detected as it is, which may result in a large error. In order to reduce such an error, magnetic encoders provided with a correcting mechanism are also proposed. However, the correcting mechanism or circuit may be complicated. For example, in a multi-rotation encoder disclosed in Japanese Patent No. 4258376, which utilizes magnetic coupling, the proportion of a magnetically coupled area is small, and the allowable transfer torque is low. Therefore, synchronization may be lost upon abrupt rotation. In addition, an error due to one magnet is detected as it is, which may result in a large error.
In an encoder used for speed measurement in a permanent-excitation electric synchronous machine disclosed in Japanese Patent Application Publication No. 2008-514906, a regular sinusoidal wave may not be obtained, and the accuracy may not be increased. In addition, a large amount of magnetic flux leaks, and only a small amount of magnetic flux reaches a magnetic sensor portion. Therefore, the S/N ratio may not be increased, which may result in a low accuracy.
Also in the structure of a magnetic pole position detector disclosed in Japanese Patent Application Publication No. 2002-62162, a large amount of magnetic flux leaks, and only a small amount of magnetic flux reaches a magnetic sensor portion. Therefore, the S/N ratio may not be increased, which may result in a low accuracy.
In a rotational angle detecting device disclosed in Japanese Patent Application Publication No. 2008-151774, a plurality of Hall sensors are used in an attempt to increase accuracy. However, distortion may still remain, and an accurate sinusoidal wave may not be obtained.
In a linear resolver disclosed in Japanese Patent Application Publication No. 2008-64537, it is difficult to make a calibration in case of a long stroke, and a small pitch may not be used. Therefore, the accuracy may not be increased.
In a method of setting an origin of a linear motor disclosed in Japanese Patent Application Publication No. 2008-289345, a small pitch may not be used, and it is difficult to obtain sinusoidal magnetic flux. Therefore, the accuracy may not be increased.
In a technology for detecting the position of a shaft motor disclosed in Japanese Patent Application Publication No. 2009-247105, a small pitch may not be used, and it is difficult to obtain sinusoidal magnetic flux. Therefore, the accuracy may not be increased.
Also in a magnetic detecting device disclosed in Japanese Patent Application Publication No. 2006-105757 and a rotation detecting device disclosed in Japanese Patent Application Publication No. 2006-58256, it is difficult to obtain sinusoidal magnetic flux. Therefore, the accuracy may not be increased.
In a position detecting device disclosed in Japanese Patent Application Publication No. 2010-71901, the position detecting accuracy depends on the accuracy of positioning magnets, which may result in a low mass productivity in consideration of variations in characteristics of the magnets.
Sinusoidal signals output from the conventional magnetic encoders are not perfectly sinusoidal, but are distorted with distortion components such as high-order harmonic components superimposed thereon. Such distortion in waveform may reduce accuracy. In order to avoid a reduction in accuracy due to waveform distortion, manufacturers have contrived to use a plurality of magnetic sensors, or to construct a system that makes a calibration using a ROM table or the like. However, such solutions are not satisfactory in terms of cost and response.
SUMMARY OF THE INVENTIONAn object of the present invention is to provide a low-cost magnetic encoder that facilitates generating sinusoidal magnetic flux and that improves resolution and interpolation accuracy.
Another object of the present invention is to provide a magnetic encoder capable of generating sinusoidal magnetic flux with little waveform distortion.
The present invention provides a magnetic encoder including: a permanent magnet array including a plurality of permanent magnets arranged such that magnetic poles having the same polarity face each other, or arranged such that the magnetic poles having different polarities face each other through a non-magnetic member interposed therebetween; a magnetic piece array including a plurality of magnetic pieces spaced from each other along the permanent magnet array; and a magnetic detector configured to detect leakage magnetic flux generated when the permanent magnet array and the magnetic piece array are displaced with respect to each other. A pitch of the plurality of permanent magnets and a pitch of the plurality of magnetic pieces are determined to form a magnetic path in which magnetic flux emitted from one of the permanent magnets in the permanent magnet array passes through one of the magnetic pieces that faces the permanent magnet or the non-magnetic member adjacent to the one permanent magnet when the permanent magnet array and the magnetic piece array are continuously displaced with respect to each other.
According to the present invention, synthesized magnetic flux synthesized from magnetic fluxes emitted from the permanent magnets included in the magnetic path is obtained when the permanent magnet array and the magnetic piece array are displaced with respect to each other. As a result, the effect of one permanent magnet can be reduced by increasing the number of the permanent magnets, allowing generation of sinusoidal magnetic flux with little distortion. By detecting leakage magnetic flux due to the generated magnetic flux using the magnetic detector, a sinusoidal signal can be obtained without requiring a special correcting circuit. As a result, it is possible to improve resolution and interpolation accuracy. The permanent magnet array is formed to include two or more permanent magnets (that may be physically separated from or integrated with each other) to provide at least two or more magnetic poles. As the number of poles for the permanent magnets in the permanent magnet array facing the magnetic piece array is increased, the detected magnetic flux approaches an accurate sinusoidal wave, thereby increasing accuracy. If the permanent magnets provide a small number (such as two) of poles, a low-cost magnetic sensor can be provided.
When the pitch P of the magnetic pieces is defined as P=360° in terms of electrical angle, the pitch τp of the permanent magnets preferably satisfies P/4<τp<P in terms of electrical angle. With this configuration, more accurate sinusoidal output can be obtained, providing a high-accuracy and high-resolution magnetic encoder.
The magnetic encoder may further include magnetic yokes disposed on both sides of each of the plurality of permanent magnets. This makes it possible to efficiently enhance flow of magnetic flux, and to reduce a repulsive force generated when the permanent magnets of the same polarity are caused to face each other, thereby facilitating manufacturing the permanent magnet array. As a matter of course, the plurality of permanent magnets in the permanent magnet array may be directly joined to each other. With this configuration, the amount of the permanent magnets can be increased.
The magnetic encoder may further include coupling yokes configured to magnetically couple the magnetic yokes located on both ends of the permanent magnet array to collect leakage magnetic flux from the permanent magnet array. In this case, the magnetic detector may be disposed to detect the leakage magnetic flux passing through the coupling yokes. Use of the coupling yokes enhances the intensity of magnetic flux detected by the magnetic detector, thereby providing higher sensitivity and improving accuracy and resolution.
The magnetic detector may be disposed at any position as long as leakage magnetic flux can be detected. For example, the magnetic detector may be disposed to face the permanent magnet array with the magnetic piece array interposed therebetween. Alternatively, the magnetic detector may be disposed to face both the magnetic piece array and the permanent magnet array. Further, the magnetic detector may be disposed adjacent to the magnetic yoke on an extension line of the permanent magnet array. Magnetic flux can be reliably detected at such positions.
One of the permanent magnet array and the magnetic piece array may be longer than the other. The plurality of magnetic pieces forming the magnetic piece array may have an integral structure in which the magnetic pieces are coupled to each other by a coupling member having a magnetic resistance higher than that of the magnetic pieces. Such an integral structure facilitates manufacture and attachment of the magnetic piece array.
The plurality of magnetic pieces forming the magnetic piece array is not limited in any way as long as sinusoidal magnetic flux is obtained, and may have a rectangular or circular profile as seen from the permanent magnet side, for example. Alternatively, the plurality of magnetic pieces forming the magnetic piece array may have a columnar shape with a circular profile as seen in a direction orthogonal to an extending direction of the magnetic piece array and to a direction toward the permanent magnet array. Further, the plurality of magnetic pieces forming the magnetic piece array may have a columnar shape with a circular profile as seen in an extending direction of the magnetic piece array. Furthermore, the permanent magnets and the magnetic yokes may be formed to have an annular shape to surround the magnetic piece array. In this case, the magnetic piece array may have an annular columnar shape with a circular profile as seen in an extending direction of the permanent magnet array. If the permanent magnets and the magnetic yokes are formed to have an annular shape, the gap between the permanent magnet array and the magnetic piece array is constant, reducing distortion caused in the sinusoidal magnetic flux.
More specifically, the magnetic piece array may be fixed to a circular plate to form an annular array along an outer peripheral surface of the circular plate. The circular plate is configured to be directly or indirectly rotated by rotation of a rotary shaft. In this case, two or more permanent magnet arrays may be disposed to arcuately extend to face the magnetic piece array. If the magnetic encoder is configured in this way, at least two sinusoidal signals at different phases can be obtained, thereby facilitating detection of the rotational position of the rotary shaft. Also in this case, when the pitch P of the magnetic pieces is defined as P=360° in terms of electrical angle, the pitch τp of the permanent magnets preferably satisfies P/4<τp≦P/2. In addition, magnetic yokes are preferably disposed on both sides of each of the plurality of permanent magnets. Further, preferably, the magnetic encoder further includes coupling yokes configured to magnetically couple the magnetic yokes located at both ends of the permanent magnet array to collect leakage magnetic flux from the permanent magnet array, and the magnetic detector is disposed to detect the leakage magnetic flux passing through the coupling yokes. With this configuration, two or more sinusoidal magnetic fluxes at different phases can be generated, providing a high-accuracy magnetic encoder. When the pitch P of the magnetic pieces is defined as P=360° in terms of electrical angle, the pitch τp of the permanent magnets may satisfy P/4<τp<P, and two magnetic piece arrays may be disposed at positions 180° away from each other in terms of mechanical angle. In addition, two magnetic detectors may be provided in correspondence with the two magnetic piece arrays, one of the two magnetic detectors being located P/4 away from an imaginary line connecting between the two magnetic piece arrays in a rotational direction of the rotary shaft, and the other of the two magnetic detectors being located P/4 away from the imaginary line in a direction opposite to the rotational direction of the rotary shaft. By adopting such an arrangement, the direction of magnetic flux that interlinks with each magnetic detector is reversed. As a result, a high S/N ratio can be obtained by connecting outputs of the two magnetic detectors to obtain a difference therebetween. In addition, it is possible to provide a more accurate magnetic encoder that can cancel the effect of an external magnetic field and that is resistant to disturbance due to an external magnetic field.
The permanent magnet array may be configured as an annular permanent magnet array centering on a rotary shaft, the magnetic piece array may be configured as an annular magnetic piece array centering on the rotary shaft, and the magnetic detector may be disposed in a region around an axis of the rotary shaft. With this configuration, if the permanent magnet array is configured such that magnetic poles having the same polarity face each other, an inner magnetic field makes as many rotations as the number of the magnetic pieces when the magnetic piece array makes one rotation, and an inner magnetic field makes as many rotations as half the number of the permanent magnets when the permanent magnet array makes one rotation. Therefore, a high-resolution magnetic sensor can be formed with a simple structure. In addition, magnetism synthesized from a plurality of permanent magnets is detected. Therefore, the effect of an error for each magnet is only marginal, thereby achieving a high precision. The annular permanent magnet array and the annular magnetic piece array may be arranged in a radial direction of the rotary shaft. Alternatively, the annular permanent magnet array and the annular magnetic piece array may be arranged in an axial direction of the rotary shaft. In such cases, a pair of magnetic detectors may be disposed such that the directions of respective magnetic fluxes detected by the pair of magnetic detectors are 180° away from each other in terms of mechanical angle. In this case, a sinusoidal signal having a high S/N ratio can be obtained by connecting output portions of the pair of magnetic detectors to obtain a difference between outputs of the pair of magnetic detectors. In addition, it is possible to obtain a more accurate magnetic encoder that can cancel the effect of an external magnetic field and that is resistant to disturbance due to an external magnetic field.
If the permanent magnet array includes a plurality of permanent magnets arranged such that magnetic poles having different polarities face each other through a non-magnetic member interposed therebetween, the non-magnetic member may be air. This means that a gap is provided between two adjacent permanent magnets, and that the gap is used as the non-magnetic member.
Magnetic encoders according to a plurality of embodiments of the present invention will be described in detail below with reference to the drawings. In the drawings referenced in the following description, in order to clarify illustration, cross sections are not hatched excluding some exceptions.
The stator 5 includes first and second permanent magnet arrays 15 and 17 each including a plurality of permanent magnets 13, and first and second magnetic detectors 19 and 21 provided in correspondence with the first and second permanent magnet arrays 15 and 17, respectively. In the embodiment, the first and second permanent magnet arrays 15 and 17 each include six permanent magnets 13 arranged such that magnetic poles having the same polarity face each other, and seven magnetic yokes 14 disposed on side surfaces of the six permanent magnets 13. The permanent magnets 13 and the magnetic yokes 14 are arranged side by side at a predetermined pitch τp in the moving direction of the mover 3. In the embodiment, the pitch τp of the permanent magnets 13 in the permanent magnet arrays 15 and 17 corresponds to the sum of the thickness of each permanent magnet 13 in the axial direction and the thickness of each magnetic yoke 14 in the axial direction. The magnetic yokes 14 are formed from a magnetic material such as iron. For example, the magnetic yokes 14 may each be formed by stacking a plurality of magnetic steel sheets made of silicon steel in the axial direction. Carbon steel, ferrite-based stainless steel, a pressed powder magnetic core, etc., may also be used as the material of the magnetic yokes 14.
The first and second magnetic detectors 19 and 21 corresponding to the first and second permanent magnet arrays 15 and 17 are integrated by a resin molding portion (not shown) in such positional relationship allowing detection of that leakage magnetic flux generated when the permanent magnet arrays 15 and 17 and the magnetic piece array 9 are displaced with respect to each other. The magnetic detectors 19 and 21 may each be a Hall sensor (that can discriminate between N pole and S pole), or a magnetic resistance element (that cannot discriminate between N pole and S pole).
If the permanent magnet arrays 15 and 17 are arranged such that the magnetic yokes 14 are located on both sides of each of the plurality of permanent magnets 13 as in the embodiment, it is possible to efficiently enhance flow of magnetic flux. Further, it is possible to reduce a repulsive force generated when the permanent magnets 13 of the same polarity are caused to face each other. This facilitates manufacturing the permanent magnet arrays 15 and 17.
The mover 3 and the stator 5 of the encoder may be installed in a mover and a stator, respectively, of a linear motor, for example, such that the permanent magnets 13 and the magnetic pieces 7 face each other with a predetermined gap between each other. In
In
The pitch τp (electrical angle) of the permanent magnets 13 preferably satisfies P/4<τp<P.
The magnetic detector 20 may be disposed at any position as long as leakage magnetic flux can be detected. For example, the magnetic detector 20 may be disposed to face the permanent magnet array 16 with a magnetic piece array 9′ interposed therebetween to detect leakage magnetic flux φ as shown in
The permanent magnet array may include two or more permanent magnets.
As shown in
As shown in
As shown in
The plurality of magnetic pieces forming the magnetic piece array may have any profile as seen from the permanent magnet side as long as sinusoidal magnetic flux is obtained. In addition, the permanent magnets forming the permanent magnet array may have any shape as long as sinusoidal magnetic flux is obtained.
A magnetic piece array 29 shown in
Magnetic pieces 37 shown in (a) and (b) of
Magnetic pieces 47 shown in (a) and (b) of
Magnetic pieces 57 shown in
Magnetic pieces 67 shown in (a) and (b) of
Permanent magnets 13′ and magnetic yokes 14′ shown in (a) and (b) of
Permanent magnets 13″ and magnetic yokes 14″ shown in (a) and (b) of
In the embodiment, the positions of the magnetic detectors 219 and 221 and the pitch P of the magnetic pieces and the pitch τp of the permanent magnets are determined such that the magnetic flux density B1 of magnetic flux detected by the magnetic detector 219 and the magnetic flux density B2 of magnetic flux detected by the magnetic detector 221 when the rotary shaft S is rotated form sinusoidal waves offset by a phase of 90° in terms of electrical angle as shown in
In
If the plurality of permanent magnets 313 are arranged such that magnetic poles having the same polarity face each other to form the annular permanent magnet array 316 as in the embodiment, inside magnetic fields (leakage magnetic flux φ) make as many rotations as the number of the magnetic pieces 307 when the magnetic piece array 309 makes one rotation. Thus, in the embodiment, the two inside magnetic fields (leakage magnetic flux φ) make 26 rotations around the axis of a rotary shaft (not shown). The principle of rotation of a magnetic field is described in detail in Japanese Patent Application No. 2010-220070 previously filed by the applicant. The magnetic detectors 320A and 320B detect the two rotating magnetic fields (leakage magnetic flux φ) to output signals.
In the configuration according to the embodiment, if the permanent magnet array 316 is rotatable and the magnetic piece array 309 is fixed, an inner magnetic field makes as many rotations as half the number of the permanent magnets 316 (that is, 25 rotations), when the permanent magnet array 316 makes one rotation.
According to the embodiment, a high-resolution magnetic sensor can be formed with a simple structure. The magnetic detectors 320A and 320B detect magnetism synthesized from magnetism from the plurality of permanent magnets 313. Therefore, the effect of an error for each magnet is only marginal, achieving a high position detecting accuracy. Also if the magnetic piece array 309 is disposed inside the permanent magnet array 316 as shown in
According to the present invention, it is possible to provide a low-cost magnetic encoder that facilitates generating sinusoidal magnetic flux and that improves resolution and interpolation accuracy.
While certain features of the invention have been described with reference to example embodiments, the description is not intended to be construed in a limiting sense. Various modifications of the example embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains, are deemed to lie within the spirit and scope of the invention.
Claims
1. A magnetic encoder comprising:
- a permanent magnet array including a plurality of permanent magnets arranged such that magnetic poles having the same polarity face each other, or arranged such that the magnetic poles having different polarities face each other through a non-magnetic member interposed therebetween;
- a magnetic piece array including a plurality of magnetic pieces spaced from each other along the permanent magnet array; and
- a magnetic detector configured to detect leakage magnetic flux generated when the permanent magnet array and the magnetic piece array are displaced with respect to each other, wherein
- a pitch of the plurality of permanent magnets and a pitch of the plurality of magnetic pieces are determined to form a magnetic path in which magnetic flux emitted from one of the permanent magnets in the permanent magnet array passes through one of the magnetic pieces that faces the permanent magnet or the non-magnetic member adjacent to the one permanent magnet when the permanent magnet array and the magnetic piece array are continuously displaced with respect to each other.
2. The magnetic encoder according to claim 1, wherein
- when the pitch P of the magnetic pieces is defined as P=360° in terms of electrical angle, the pitch τp of the permanent magnets satisfies P/4<τp≦P/2.
3. The magnetic encoder according to claim 1, further comprising:
- magnetic yokes disposed on both sides of each of the permanent magnets.
4. The magnetic encoder according to claim 3, further comprising:
- coupling yokes configured to magnetically couple the magnetic yokes located at both ends of the permanent magnet array to collect leakage magnetic flux from the permanent magnet array, wherein
- the magnetic detector is disposed to detect the leakage magnetic flux passing through the coupling yokes.
5. The magnetic encoder according to claim 1, wherein
- the magnetic detector is disposed to face the permanent magnet array with the magnetic piece array interposed therebetween.
6. The magnetic encoder according to claim 1, wherein
- the magnetic detector is disposed to face both the magnetic piece array and the permanent magnet array.
7. The magnetic encoder according to claim 3, wherein
- the magnetic detector is disposed adjacent to the magnetic yoke on an extension line of the permanent magnet array.
8. The magnetic encoder according to claim 1, wherein
- the permanent magnet array is longer than the magnetic piece array.
9. The magnetic encoder according to claim 1, wherein
- the plurality of magnetic pieces forming the magnetic piece array have an integral structure in which the magnetic pieces are coupled to each other by a coupling member having a magnetic resistance higher than that of the magnetic pieces.
10. The magnetic encoder according to claim 1, wherein
- the plurality of magnetic pieces forming the magnetic piece array have a rectangular or circular profile as seen from the permanent magnet side.
11. The magnetic encoder according to claim 1, wherein
- the plurality of magnetic pieces forming the magnetic piece array have a columnar shape with a circular profile as seen in a direction orthogonal to an extending direction of the magnetic piece array and to a direction toward the permanent magnet array.
12. The magnetic encoder according to claim 1, wherein
- the plurality of magnetic pieces forming the magnetic piece array have a columnar shape with a circular profile as seen in an extending direction of the magnetic piece array.
13. The magnetic encoder according to claim 3, wherein
- the permanent magnets and the magnetic yokes are formed to have an annular shape to surround the magnetic piece array.
14. The magnetic encoder according to claim 3, wherein
- the permanent magnets and the magnetic yokes are formed to have an annular shape to surround the magnetic piece array which has an annular columnar shape with a circular profile as seen in an extending direction of the permanent magnet array.
15. The magnetic encoder according to claim 1, wherein
- the magnetic piece array is fixed to a circular plate to form an annular array along an outer peripheral surface of the circular plate, the circular plate being configured to be directly or indirectly rotated by rotation of a rotary shaft, and two or more permanent magnet arrays are disposed to arcuately extend to face the magnetic piece array.
16. The magnetic encoder according to claim 15, wherein
- when the pitch P of the magnetic pieces is defined as P=360° in terms of electrical angle, the pitch τp of the permanent magnets satisfies P/4<τp<P.
17. The magnetic encoder according to claim 15, further comprising:
- magnetic yokes disposed on both sides of each of the permanent magnets.
18. The magnetic encoder according to claim 17, further comprising:
- coupling yokes configured to magnetically couple the magnetic yokes located at both ends of the permanent magnet array to collect leakage magnetic flux from the permanent magnet array, wherein
- the magnetic detector is disposed to detect the leakage magnetic flux passing through the coupling yokes.
19. The magnetic encoder according to claim 17, wherein:
- when the pitch P of the magnetic pieces is defined as P=360° in terms of electrical angle, the pitch τp of permanent magnets satisfies P/4<τp<P;
- two magnetic piece arrays are disposed at positions 180° away from each other in terms of mechanical angle; and
- two magnetic detectors are provided in correspondence with the two magnetic piece arrays, one of the two magnetic detectors being located P/4 away from an imaginary line connecting between the two magnetic piece arrays in a rotational direction of the rotary shaft, and the other of the two magnetic detectors being located P/4 away from the imaginary line in a direction opposite to the rotational direction of the rotary shaft.
20. The magnetic encoder according to claim 1, wherein:
- when magnetic poles having the same polarity face each other, the permanent magnet array is configured as an annular permanent magnet array centering on a rotary shaft, and the magnetic piece array is configured as an annular magnetic piece array centering on the rotary shaft; and
- the magnetic detector is disposed in a region around an axis of the rotary shaft.
21. The magnetic encoder according to claim 20, wherein
- the annular permanent magnet array and the annular magnetic piece array are arranged in a radial direction of the rotary shaft.
22. The magnetic encoder according to claim 20, wherein
- the annular permanent magnet array and the annular magnetic piece array are arranged in an axial direction of the rotary shaft.
23. The magnetic encoder according to claim 21, wherein
- a pair of magnetic detectors are disposed such that the directions of respective magnetic fluxes detected by the pair of magnetic detectors are 180° away from each other in terms of mechanical angle.
24. The magnetic encoder according to claim 1, wherein
- the non-magnetic member is air.
Type: Application
Filed: Oct 26, 2012
Publication Date: May 2, 2013
Patent Grant number: 8928313
Applicant: SANYO DENKI CO., LTD. (Tokyo)
Inventor: SANYO DENKI CO., LTD. (Tokyo)
Application Number: 13/661,247
International Classification: G01B 7/30 (20060101);